Method and device for measuring the qualities of a multiphase fluid

ABSTRACT

A method and a device for measuring the qualities of a fluid including at least two liquids, the two liquids having at least one characteristic intrinsic factor relative to microwaves for a given frequency, such as the loss factor. The characteristic of the fluid is measured by a microwave flux for at least two different microwave frequencies, and, with the values of the factors of the intrinsic characteristics of each of the fluids at the two frequencies being known, the respective quantities of the two liquids are determined. The invention applies in particular to measuring the concentrations and flowrates of various phases in a multiphase fluid such as a petroleum effluent having an aqueous phase.

The invention relates to a device and a method for measuring thequalities of a fluid, comprising at least two liquids, by means inparticular of microwaves, whereby these qualities may be the quantitiesof the two liquids, their speed, their flowrate, or the quantity orspeed of a gas contained in the fluid.

The invention applies in particular to production of hydrocarbonscontaining a multiphase gas-liquid mixture, which production may inparticular, but not exclusively, be effected in a setting to whichaccess is difficult, for example at a wellhead or the head of anundersea pipeline, or in a virgin forest.

The invention also applies to the chemical and oil industries, or ingeneral to all industries employing fluids containing at least twoliquids, whether or not these liquids are miscible.

In oil production in particular, the attempt is generally made todiscover the quantities of flowrates of the three components--water,liquid hydrocarbons, and gas--which constitute three separate phaseswhose qualities are difficult to analyze continuously.

It is known that neutron bombardment, gamma radiation (dichromic),microwaves, and nuclear magnetic resonance may be used to measure theconcentration of a single liquid in a fluid comprising several liquids.Moreover, classical measuring techniques employing microwaves candetermine only the concentration of a liquid having high absorption bycomparison to another liquid with which it is mixed.

The method and the device according to the present invention allow theabsolute and relative quantities of at least two liquids contained in afluid to be measured.

In carrying out the measurements, at least one intrinsic characteristicof the fluids relative to microwaves is used. This characteristic, whichmay be absorption, reflection, diffusion, diffraction, polarization,phase shift of microwaves, is defined by a factor. This factor dependson the characteristics of the microwave beam, for example the frequencyor frequencies of this beam, and of the fluid or more generally the bodytraversed. Advantageously, the characteristic measured is absorption andthe factor is, for example, the microwave absorption factor which iscalled a microwave loss factor or loss factor. This factor can bemeasured by transmission or by transmission and reflection.

Microwaves are understood to be waves with a frequency of between 0.1and 1000 GHz, but advantageously for implementation of the method orconstruction of the device according to the invention, the microwavefrequency used is between 1 and 100 GHz, or between 10 and 100 GHz.

The invention proposes a method for measuring the qualities of a fluidcomprising at least two liquids, the two liquids having at least onefactor of a characteristic intrinsic to microwaves for a givenfrequency, one of the two liquids being a first liquid and the otherliquid being a second liquid. This method is characterized in particularby measuring the characteristic of the fluid by a microwave flux for atleast two different microwave frequencies, and, with the values of thefactors of the intrinsic characteristics of each of the fluids at thetwo frequencies being known, the respective quantities of the twoliquids are determined, one of the two frequencies being hereinaftercalled the first frequency and the other the second frequency.

Other frequencies different from the first and second frequencies,having either the properties of the first frequency or those of thesecond frequency, may refine the accuracy of the measurements sought.

The factor of the intrinsic characteristic may be the microwave lossfactor, and the intrinsic characteristic will be absorption with respectto microwaves.

The fluid may comprise gas.

In the case where, for the first frequency, the first liquid has anintrinsic characteristic factor that is negligible by comparison withthe intrinsic characteristic factor of the second liquid, the secondfrequency being a frequency at which the intrinsic characteristic factorof the first liquid is not negligible by comparison with the intrinsiccharacteristic factor of the second liquid, the quantity of the secondliquid may be determined by measuring the microwave flux characteristicthrough the liquid at the first frequency, neglecting the intrinsiccharacteristic factor of the first liquid by comparison to thecharacteristic factor of the second liquid.

When the second liquid includes an aqueous compound, the first frequencymay be close to 21 GHz and the second frequency may be remote from 21GHz. "Remote frequency" is understood to be the frequency above whichthe instrument measuring the microwave flux characteristic is capable ofdetecting a given quantity of the first liquid.

The first liquid may include a hydrocarbon compound.

If the fluid is moving in one direction,

the variation in frequency of a microwave flux with an initial frequencytraversing the fluid in a direction not perpendicular to the directionof movement can be measured, the initial frequency being a frequency forwhich the intrinsic characteristic factor of the first or second liquidis negligible by comparison to the intrinsic characteristic factors ofthe second or first liquid respectively, and

the velocity of the liquid whose intrinsic characteristic factor ispreponderant can be measured.

When the fluid is moving,

the change as a function of time in the amplitude of a microwave fluxhaving at least one initial frequency and traversing the fluid in adirection not perpendicular to the direction of movement can bedetermined, the initial frequency being a frequency for which theintrinsic characteristic factors of the first and second fluid are ofthe same order of magnitude, then

a frequency analysis may be made of the changes in the amplitude inorder to establish at least one frequency variation, and

from the variation or variations in initial frequency and thecomposition of the fluid, the velocity of each of the liquids of thefluid may be determined.

When the fluid includes a moving gas, the gas having particles withwhich factor of an intrinsic characteristic relative to microwaves isassociated,

at least part of the gas and the liquids can be separated,

the variation in frequency of microwave flux having an initial frequencyand traversing the gas part in a direction not perpendicular to themovement can be measured, and

the velocity of the gas can be determined from the variation infrequency.

The initial frequency of the microwave flux traversing the fluid may bechosen such that the intrinsic characteristic factor of the first orsecond liquid is negligible by comparison to the intrinsiccharacteristic factor of the second or the first liquid, respectively.

The fluid could be disposed in a volume having two parallel faces, and

the characteristic of the microwave flux traversing at least part of theparallel faces can be detected or measured.

The invention also proposes a device for measuring the qualities of afluid.

This device is characterized in particular by having a volume to holdthe liquid which has two parallel windows transparent to microwaves, andby having a microwave transmitter for microwaves that pass through oneor the first of the windows, and a microwave receiver sensitive tomicrowaves that have passed through the first and the second window.

These windows may be disposed on either side of the volume such that thevolume is traversed through and through, for example in directtransmission measurements.

These windows may be disposed on the same side of the volume, forexample when measurements are to be made by reflection ortransmission-reflection.

The windows may be made of a composite material transparent tomicrowaves.

When the fluid is flowing, the volume of the device may be a section ofpipe.

The invention is based in particular on the following principles:

the intrinsic characteristic factor, such as the loss factor forabsorption, depends both on the material, such as the fluid, and on thefrequency at which this factor is measured;

the factor generally varies a great deal with the frequency and has anextreme which is a maximum for a given frequency and which depends onthe material. Hence, when a fluid which has at least two liquids whoseintrinsic characteristic factors are different, is to be analyzed, acharacteristic such as absorption is measured for at least twofrequencies chosen to permit discrimination between the various factorsof the characteristics at these two frequencies. Preferably, one or,even better, both frequencies are chosen as being frequencies at whichone of the factors is negligible by comparison to the other.

For example, in a mixture of water and hydrocarbon oil, the measuringfrequency is chosen at about 21 GHz since, for the absorption beingmeasured, the loss factor of water is about 30 to 40, while the lossfactor of oil is bout 10⁻⁶. In this case, the ratio between theabsorption factors is about 3·10⁷.

Hence, measurement of absorption at this frequency furnishes the watercontent of the fluid.

Once its water content is known, one need then only measures theabsorption of the fluid at another frequency and determine the oilcontent.

The invention will be better understood and its advantages will emergeclearly from reading the following description illustrated by theattached figures, wherein:

FIG. 1 shows schematically the operating principle of the measuringdevice according to the invention; and

FIG. 2 illustrates a particular embodiment of the device in a crosssection in the direction perpendicular to the fluid flow.

The diagram of FIG. 1 shows the operating principle of the device formeasuring the qualities of a fluid flowing through a reinforced tube 1which is connected both upstream and downstream to a diffuser 2 fortransition from a circular cross section to a rectangular cross section,this diffuser having contant cross section, and a rectangular upstreamconverging section 3 or a rectangular downstream diverging section 4.

Upstream of a given point means the zone that an element of material (ofthe fluid) traverses during an industrial process before reaching thepoint in question, while on the contrary downstream means the zonereached after the point in question.

Upstream is to the left of the figure and downstream is to the right.The arrow in converging section 3 indicates the flow direction of thefluid.

Acquisition and processing of measurements may be broken down intoseveral stages corresponding to separate measuring chains A, B, C, D, E,F. The elements of one measuring chain are reference by the same letter.

Chains A to E are measuring chains, using microwaves, each of which hasa microwave transmitter 5, each of the transmitters transmitting at agiven frequency or in a fixed frequency range, a microwave detector 6,which can be a microwave amplitude detector 6A. 6C or a microwaveamplitude and frequency detector 6B, 6D, 6E, and an amplifier 7 of thesignals of detector 6.

Chains B, C, E allow the velocity of the various fluid components to bemeasured by the Doppler effect. These chains in addition have ananalyzer comparator 8 designed to compare the frequency of the amplifiedsignal to the frequnecy of the signal emitted by transmitter 5 andpossibly designed to perform a frequency analysis, for example toanalyze a Fourier series, to determine the velocity of several fluids byshifts of the transmission frequency.

Chain A allows the water cross section in tube 1 to be determined byanalyzing the absorption of the microwave flux at the frequency of 21GHz.

Chain B allows the velocity of water in tube 1 to be determined byanalyzing the shift in frequency of the microwave flux initially at thefrequency 21 GHz.

The frequency of 21 GHz is chosen to isolate the behavior of the water.Once the water cross section and the velocity of the water are known,the volume flowrate of the water can be determined.

Chain C allows a relationship to be determined between the velocity ofthe water and the velocity of the oil by analyzing the frequency shiftsof the microwave flux initially at a terminal frequency essentiallybetween 35 and 80 GHz. Once this relationship is known, with thevelocity of water known from chain B, the velocity of the oil can bedetermined.

Chain D allows the liquid (oil plus water) cross section in tube 1 to bedetermined by absorption analysis of the microwave flux at a terminalfrequency between 35 and 80 GHz. From this, the oil velocity, the watercross-section measurement, and the oil cross-section the quantity of oiltransported in the multiphase effluent can be deduced.

When the gas is completely separated from the liquids, chain E allowsthe velocity of the gas to be determined by a Doppler effect, from theparticles entrained by the gas. The measurement is performed on theupper part of the flow, in which gas is found preferentially.

When the gas is dispersed within the liquid in the form of bubbles, thevelocity of the gas is the same as the velocity of the liquids,disregarding the vertical component of the velocity, which is generallynegligible relative to the horizontal component of the velocity.

When the gas passage cross section has been calculated from thedifference between the fluid passage cross section in the tube and thepassage cross sections of the water or oil previously determined, thevolume flowrate of the gas is calculated by multiplying the passagecross section by the average velocity of the gas.

When the measurement are being made, it is preferable to dispose thetube in such a way that its lengthwise axis is horizontal and thelongest dimension of its cross section is vertical, which is the case inFIG. 2 where the plane of the figure is vertical.

The type of flow with separated gas or bubbles inside the liquid can bedetermined by disposing a multiple chain E having a series of microwavesensors disposed velocity along the wall of the measuring section. Thesensors would be of the second frequency type, sensitive only to thefrequency of the liquids. The velocity of the free gas would be givenonly at the height corresponding to its presence in the measuringsection. The information would be processed by a computer program.

Measuring chain F uses ultrasound to measure the density of the gas inthe fluid, which is located in the upper part of the flow.

An ultrasound signal is transmitted by transmitter 9 and passes througha first wall in of tube 1, the gas, then a second wall of tube 1 beforebeing detected by a sensor 10 furnishing an electrical signal amplifiedby amplifier 11. The ultrasound signal is transmitted in the form of apulse, and comparator 12 allows the time necessary for propagation ofthe sound wave to be established, whereby this time depends on thedensity of the gas and the temperature value furnished by heat sensor13. The mass flowrate of the gas can then be obtained from the volumeflowrate, previously calculated, and the density.

All the information from measuring chains A to E and sensor 13 isprocessed by a computer 14 in order to furnish instant or cumulativevalues of the cross sections of the various fluids components at thevelocity of these components.

Transmitters 5B, 5C, 5E, and detectors 6B, 6C, 6E are disposedrespectively opposite each other, but their transmission or receptionaxis is inclined by 45° with respect to the flow axis in order todetermine the velocity of the various components of the fluid.

Once the flowrate, temperature, and pressure (easy to obtain) are known,the density of the oil and water, and hence the mass flowrates of theoil and water, of an oil deposit, for example, can be determined by thePVT method used in the oil and chemical industries.

FIG. 2 illustrates a particular embodiment of the device according tothe invention in cross section in the direction perpendicular to thefluid flow direction.

Tube 1, which appears in oblong rectangular form, is made of a materialtransparent to microwaves such as a resin-glass composite, and isresistant to the pressure differences between the inside and the outsideof the pipe. This tube 1 is reinforced by ribs 15 disposed all around itat rectangular intervals. The rectangular shape of tube 1 was designedto permit natural separation of the gas and the liquids, and to allowcorrect measurement of the passage cross sections of the variousliquids, since, when absorption is measured, it depends exponentiallyand nonlinearly on the thickness of the material traversed by theelectromagnetic microwave flux (Beer-Lambert law).

The various transmitters and detectors are disposed along the tubebetween the ribs, and the other elements (amplifiers, analyzers,comparators) are disposed in a watertight tank 16 to allow immersionunder the sea. This tank also allows the effluents to be contained ifthe measuring tube transparent to microwaves should deteriorate. Tube 1has two flanges 17 for easy installation on a pipe.

In offshore oil production, tank 16 is connected to an electrohydraulicand electronic control module to provide the interface with theaforementioned computer which is at the surface of the sea.

The computer could also be built into the submerged tank, with thesignals produced by the computer being multiplexed to be transmitted tothe surface.

The measuring tube may be placed near an offshore production well inorder to determine the composition of its effluent at the well outlet,as well as the flowrates of the effluents under local pressure andtemperature conditions.

A nonlimitative embodiment of the method according to the invention willbe indicated below.

Absorption and transmission of an electromagnetic wave, e.g. amicrowave, through a sample, are governed by the Beer-Lambert law:##EQU1## where: I=intensity of wave flux leaving the sample

Io=intensity of wave flux entering the sample

Ka=molar absorption factor of the material of which the sample is made

h=thickness of the sample

A=absorption.

Ka is a property of the sample material; it can be determined from thepermittivity ε" in the complex plane. ε" is also called loss factor.##EQU2## where: f=measuring frequency

k=extinction coefficient

n=refractive index of sample material

V=volume fraction of sample material

c=velocity of electromagnetic waves in vacuum,

[C]=molar concentration of material in sample.

ε" varies according to frequency and is measurable. The variations andamplitude of ε" depend on the type of material.

When total absorption A_(T) is measured through walls (P) of a measuringtube, as described above, containing a fluid composed of water (W), oil(H), and gas (G), the sum of the absorptions of the components ismeasured, namely:

    A.sub.T =A.sub.P +A.sub.W +A.sub.H +A.sub.G.

If the total absorption A_(T1) is measured at a frequency (index 1) ofabout 21 GHz (relaxation frequency of water), the water absorption willbe vary large relative to that of the oil and gas, with the loss factorsof water and oil being ε"_(W1) =40 and ε"_(H1) =2×10⁻⁶, respectively.

Total absorption will be: A_(T1) =A_(P1) +A_(W1).

If the total absorption A_(T2) is measured at another frequency (index2) for which the absorption contrast between the water and oil is small,for example at frequency f₂ =80 GHz (ε"_(W2) =4 and ε"_(H2) =2) theabsorption of the gas will still be negligible and hence A_(T2) =A_(P2)+A_(W2) +A_(H2).

Since absorption A_(P1) of the tube walls at frequency 1 has beenmeasured previously, A_(W1) is determined from the value of A_(T1) and,since Ka_(W1) or ε"_(W1) is known, the quantity of water or thickness ofwater h_(W) can be determined by the relation: ##EQU3##

Once the quantity of water is known, and with a measurement tableavailable for the absorption coefficient of water Ka or loss factor ε"of water as a function of frequency, the absorption of water A_(W2) atfrequency 2 is determined.

Now that terms A_(T2), A_(P2), and A_(W2) are known, A_(H2) isdetermined by the formula:

    A.sub.H2 =A.sub.T2 -A.sub.P2 -A.sub.W2

and thus, since Ka_(H2) or ε"_(H2) is known, we have the quantity of oilor thickness of oil h_(H) ##EQU4## in the measuring section.

The quantity of gas in the measuring section is the volume not occupiedby water or oil h_(G) =h_(T) -(h_(H) +h_(W)), h_(T) being the thicknessof the measuring section.

Hence, the proportions of water, oil, and gas are defined respectivelyas follows:

    h.sub.W /h.sub.T,h.sub.H /h.sub.T,h.sub.G /h.sub.T.

We claim:
 1. A method of determining a quality of a fluid moving in afirst direction within a tube means of known cross-section, the fluidincluding a first liquid and a second liquid, each liquid having afrequency-dependent intrinsic characteristic response to microwaveenergy, said method comprising the steps of:(a) passing microwave energyof a first frequency through the fluid-containing tube means in a seconddirection not perpendicular to the first direction, the first frequencybeing a frequency for which the value of the intrinsic characteristic ofthe first liquid is large in comparison to the value of the intrinsiccharacteristic of the second liquid and for which the value of theintrinsic characteristic of the tube means is known; (b) determining thevalue of the intrinsic characteristic of the microwave energy of thefirst frequency after passage thereof through the fluid-containing tubemeans; (c) determining the change in frequency of the microwave energyof the first frequency after passage thereof through thefluid-containing tube means; (d) passing microwave energy of a secondfrequency through the fluid-containing tube means in the seconddirection, the second frequency being a frequency for which thedifference between the value of the intrinsic characteristic of thefirst liquid and the value of the intrinsic characteristic of the secondliquid is small and for which the value of the intrinsic characteristicof the tube means is known; (e) determining the value of the intrinsiccharacteristic of the microwave energy of the second frequency afterpassage thereof through the fluid-containing tube means; (f) determiningthe change in frequency of the microwave energy of the second frequencyafter passage thereof through the fluid-containing tube means; (g)determining the cross-section of the first liquid within the tube meansfrom the known value of the intrinsic characteristic of the tube meansto the microwave energy of the first frequency and the result of step(b); (h) determining the cross-section of the combined first and secondliquids within the tube means from the known value of the intrinsiccharacteristic of the tube means to the microwave energy of the secondfrequency and the result of step (e); (i) determining the velocity ofthe first liquid within the tube means from the result of step (c); (j)determining the relationship between the velocity of the first liquidand the velocity of the second liquid within the tube means from theresult of step (f); (k) determining the cross-section of the secondliquid within the tube means from the results of steps (g) and (h); (l)determining the velocity of the second liquid within the tube means fromthe results of steps (i) and (j); (m) determining the volume flow rateof the first liquid within the tube means from the results of steps (g)and (i); and (n) determining the volume flow rate of the second liquidwithin the tube means from the results of steps (k) and (l).
 2. A methodas claimed in claim 1, wherein the intrinsic characteristic is theabsorption of microwave energy.
 3. A method as claimed in claim 1 or 2,wherein step (b) includes neglecting the change in the intrinsiccharacteristic of the microwave energy of the first frequency due to thesecond liquid.
 4. A method as claimed in claim 1, wherein the firstliquid has an aqueous component, and wherein the first frequency is afrequency in the order of about 21 GHz and the second frequency is afrequency remote from 21 GHz.
 5. A method as claimed in claim 1, whereinthe fluid further includes a moving gas having particles with anintrinsic characteristic response to microwave energy, and wherein saidmethod further comprises the steps of:(o) separating at least a part ofthe gas from the liquids; (p) passing microwave energy of a knownfrequency through the separated gas in the second direction, the knownfrequency being a frequency for which the value of the intrinsiccharacteristic of the tube means is known; (q) determining the change infrequency of the last-named microwave energy after passage thereofthrough the separated gas; (r) determining the cross-section of the gasfrom the known cross-section of the tube means and the result of step(h); (s) determining the velocity of the gas within the tube means fromthe result of step (g); and (t) determining the the volume flow rate ofthe gas within the tube means from the results of steps (r) and (s). 6.A method as claimed in claim 5, wherein the intrinsic characteristic isthe absorption of microwave energy.
 7. A method as claimed in claim 5,wherein the first liquid has an aqueous component, and wherein the firstfrequency is a frequency in the order of about 21 GHz and the secondfrequency is a frequency remote from 21 GHz.
 8. A method as claimed inclaim 5, wherein the tube means includes a section having two parallelfaces, and wherein each of step (a) and step (d) comprises passingmicrowave energy through the parallel faces.
 9. A method of determiningthe volume flowrates of first and second liquids in a fluid moving in afirst direction within a tube means of known cross-section, each liquidhaving a frequency-dependent absorption characteristic response tomicrowave energy, said method comprising the steps of:(a) passingmicrowave energy of a first frequency through the fluid-containing tubemeans, the first frequency being a frequency for which the absorptioncharacteristic of the first liquid is large in comparison with theabsorption characteristic of the second liquid and for which theabsorption characteristic of the tube is known; (b) determining theabsorption of the microwave energy of the first frequency by the firstliquid during passage of the microwave energy of the first frequencythrough the fluid-containing tube means; (c) passing microwave energy ofthe first frequency through the fluid-containing tube means in a seconddirection not perpendicular to the first direction; (d) determining thechange in frequency of the last-named microwave energy after passagethreof through the fluid-containing tube means; (e) passing microwaveenergy of a second frequency through the fluid-containing tube means,the second frequency being a frequency for which the difference betweenthe absorption characteristic of the first liquid and the absorptioncharacteristic of the second liquid is small and for which theabsorption characteristic of the tube means is known; (f) determiningthe absorption of the microwave energy of the second frequency by firstand second liquids during passage of the microwave energy of the secondfrequency through the fluid-containing tube means; (g) passing microwaveenergy of the second frequency through the fluid-containing tube meansin the second direction; (h) determining the change in frequency of thelast-named microwave energy after passage thereof through thefluid-containing tube means; (i) determining the cross-section of thefirst liquid within the tube means from the known absorptioncharacteristic of the tube to the microwave energy of first frequencyand the result of step (b); (j) determining the velocity of the firstliquid within the tube means from the result of step (d); (k)determining the cross-section of the combined first and second liquidswithin the tube means from the known absorption characteristic of thetube means to the microwave energy of the second frequency and theresult of step (f); (l) determining the relationship between thevelocity of the first liquid and the velocity of the second liquid fromthe result of step (h); (m) determining the cross-section of the secondliquid within the tube means from the results of steps (i) and (k); (n)determining the velocity of the second liquid from the results of steps(j) and (l); (o) determining the volume flowrate of the first liquidwithin the tube means from the results of steps (i) and (j); and (p)determining the volume flowrate of the second liquid within the tubemeans from the results of steps (m) and (n).
 10. A method as claimed inclaim 9, wherein step (b) includes neglecting the absorption of themicrowave energy of the first frequency by the second liquid.
 11. Amethod as claimed in claim 9, wherein the first liquid has an aqueouscomponent, and wherein the first frequency is a frequency in the orderof about 21 GHz and the second frequency is a frequency remote from 21GHz.
 12. A method as claimed in claim 9, wherein the fluid includes amoving gas having particles with an absorption characteristic tomicrowave energy, said method further comprising the steps of separatingat least a part of the gas from the liquids, passing microwave energy ofa known frequency through the separated gas; determining the change infrequency of the last-named microwave energy after passage through thegas; and determining the velocity of the gas from the last-named changein frequency.
 13. A method as claimed in claim 9, wherein the tube meansincludes a section having two parallel faces, and wherein each of step(c) and step (e) comprises passing microwave energy through the parallelfaces.
 14. Apparatus for determining a quality of a fluid moving in afirst direction, the fluid including a first liquid and a second liquid,each liquid having a frequency-dependent intrinsic characteristicresponse to microwave energy, said apparatus comprising:(a) a tubemember, having two parallel windows transparent to microwave energy andhaving a known cross-section, for passage of the moving fluidtherethrough; (b) first transmitting means for transmitting microwaveenergy of a first frequency through the tube member windows and themoving fluid; the first frequency being a frequency for which the valueof the intrinsic characteristic of the first liquid is large incomparison to the value of the intrinsic characteristic of the secondliquid and for which the intrinsic characteristic of the tube memberwindows is known; (c) first determining means positioned to receivemicrowave energy from said first transmitting means after passagethereof through said tube member windows and the fluid, for determiningthe value of the cross-section of the first liquid within said tubemember; (d) second transmitting means for transmitting microwave energyof the first frequency through the tube member windows and the movingfluid in a second direction not perpendicular to the first direction;(e) second determining means positioned to receive microwave energy fromsaid second transmitting means after passage thereof through said tubemember windows and the fluid, for determining the value of the velocityof the first liquid within said tube member; (f) third transmittingmeans for transmitting microwave energy of a second frequency throughthe tube member windows and the moving fluid in the second direction;the second frequency being a frequency for which the difference betweenthe value of the intrinsic characteristic of the first liquid and thevalue of the intrinsic characteristic of the second liquid is small andfor which the value of the intrinsic characteristic of the tube memberwindows is known; (g) third determining means positioned to receivemicrowave energy from said third transmitting means after passagethereof through said tube member windows and the fluid, for determiningthe relationship between the velocity of the first liquid and thevelocity of the second liquid within said tube member; (h) fourthtransmitting means for transmitting microwave energy of the secondfrequency through the tube member windows and the moving fluid; (i)fourth determining means positioned to receive microwave energy fromsaid fourth transmitting means after passage thereof through said tubemember windows and the fluid, for determining the value of thecross-section of the combined first and second liquids within said tubemember; (j) computing means, coupled to said first, second, third, andfourth receiving means and said second and third transmitting means, andresponsive to the determined values and relationship for computing thevolume flowrates of the first and second liquids.
 15. Apparatus asclaimed in claim 14, wherein said tube member windows are made of acomposite material.
 16. Apparatus as claimed in claim 14, wherein saidtube member is a section of pipe.
 17. Apparatus as claimed in claim 14,wherein the first frequency is a frequency in the order of about 21 GHzand the second frequency is a frequency remote from 21 GHz.